Close NNTP connections left idle for NNTP_IDLE_CLOSE_SECONDS

Eweka drops quiet connections, so the next command on one failed
("Spot inspection failed: NNTP connection closed"), and idle connections
held account slots that downloads could use. The coordinator pool now
closes free connections (and the plain one) on its sync once idle for
60 s; dry prober lanes, the spotter between rounds and the spot inspector
with nothing to inspect close theirs too, reconnecting when work comes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

start/env.ts

@@ -50,6 +50,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   NNTP_HEDGE_AFTER_SECONDS: Env.schema.number.optional(),
   NNTP_HEDGE_CONNECTIONS: Env.schema.number.optional(),
   NNTP_CONNECTION_MAX_MINUTES: Env.schema.number.optional(),
+  NNTP_IDLE_CLOSE_SECONDS: Env.schema.number.optional(),
   NNTP_STALL_KB_PER_MINUTE: Env.schema.number.optional(),
 
   // Newznab API key (empty: no key needed)